Abstract : A series of flight tests were conducted at the Federal Aviation Administration Technical Center to compare the range and azimuth resolution capabilities of the Mode S (formerly the Discrete Address Beacon System (DABS)), in the Air Traffic Control Radar Beacon /System (ATCRBS) mode, to an existing Automated Radar Terminal System (ARTS) III. The minimum achievable range and azimuth separation of two aircraft, without garbling of either aircraft's A-Code, was determined for both systems. The resolution results were compared to positional aircraft separation data, collected concurrently by a precision Range Instrumentation System, to determine the relationship between A-Code garbling and aircraft separation. The flight test results indicate that the 89 percent beacon resolution achieved the correlated-only Mode S sensor has the best overall resolution in the aircraft separation intervals of 0 deg to 2 deg in azimuth and o to 10,000 feet in range. The combined correlated and uncorrelated Mode S reports were 80 percent resolved, whereas, the ARTS reports for the same aircraft separation intervals were 62 percent resolved. The minimum achievable range separation, without garbling, was approximately 10,000 feet for both systems. The azimuth separation was 2 deg for the Mode S system and 3.2 for the ARTS system.